There are several options that are acceptable. Really comes down to what you prefer. I have seen the PW, GW, SW and LW all the same length for some, especially when the clubs are shorter than standard. You could make the PW and GW the same lengths and you could progress the SW and LW 1/4″ shorter if you just want some progression of length. Most of our PW’s and GW’s within the same set are the same head weight, so by making them the same length the swing weights would be consistent. SW’s and LW’s are usually 7-10 grams heavier than PW’s or GW’s, so if you make them the same length as the PW and GW, the swing weight will be heavier. That is OK, but just be aware if your PW and GW are D2 for example, the SW and LW (depending on the model) could be D5-D7. Making the SW and LW 1/4″ shorter would lessen that gap in swing weight a little. Also, analyze what kind of shots you hit the majority of the time with the SW and LW. If you find yourself gripping down and hitting more 3/4 shots or less on those clubs, then that would indicate to me you could make them shorter than the PW and GW.
Again, it is really about what you prefer in the wedges from a length and head feel standpoint. There is no standard progression once you get into the wedges.